1. Field of the Invention
The present invention relates generally to reconstituting fuel assemblies for nuclear reactors and, more particuarly, is concerned with a precision internal tube cutter especially adapted for use in circumferentially cutting a guide thimble adjacent its region of attachment to the top nozzle of the fuel assembly without producing any burrs, observable roughness or distortion of the guide thimble at its severed upper end which facilitates reconstitution of the fuel assembly.
2. Description of the Prior Art
Nuclear reactors of the type utilized in power generation incorporate a large number of fuel assemblies arranged in a suitable configuration. Each fuel assembly includes numerous fuel rods, each of which comprises a stack of fuel pellets housed inside a hollow tube sealed by end plugs. The fuel rods are held in an organized array by a "skeleton" formed of grids spaced at intervals lengthwise along the fuel rods, control rod guide thimbles interconnecting the grids and extending beyond the opposite ends of the fuel rods, and top and bottom nozzles attached to the opposite ends of the thimbles. The guide thimbles may each include a tubular body which at its opposite ends is surrounded by and connected to sleeves which interconnect respective nozzles and adjacent grids.
The metal tubes of the fuel rods thus constitute the primary containment boundary for the radioactive nuclear fuel therein. During initial installation and periodic shutdown of the reactor for refueling, the fuel rods are inspected for damage which might allow leakage and thus contamination of the primary coolant. Fuel rods may develop cracks, pin holes, or other defects during service which must be detected, but which is difficult because they are part of the integral fuel assemblies and because irradiated fuel assemblies must be submerged in neutron absorbing liquid such as water. When such defects are detected, it is often necessary to remove and replace the entire fuel assembly in view that attempted repair often causes further damage which renders it unfit for further use. This, of course, is extremely time-consuming and expensive. Fuel assemblies incorporating special design features to facilitate reconstitution have been developed; however, such features add more expense and complication, and are unnecessary if the need for reconstitution does not arise.
As a result, there are many partially spent fuel assemblies stored in reactor plant spent fuel pools which have sustained damage and/or contain leaking fuel rods. These assemblies, which usually have weld-attached top nozzles, are unacceptable for reinsertion into the reactors by reason of their defective fuel rods. If the weld-attached nozzles on the assemblies could be removed in such a manner as to permit reattachment of replacement nozzles, the assemblies could be salvaged and reinserted into the reactors for the balance of their normal design life. Other potential advantages could be realized if replacement nozzles could be reattached to irradiated assemblies, such as the rearrangement or replacement of selected fuel rods for extended burnup.
A method has been developed by Westinghouse, the assignee hereof, for remounting top nozzles on irradiated fuel assemblies, the method being described in the first patent application cross-referenced above. However, to remount nozzles it is necessary to have a reliable, precision means of cutting the guide thimbles without producing chips, burrs, or tube distortions that could interfere with mounting the close tolerance passageways in the top nozzle adapter plate over the cut upper ends of the guide thimbles. The close spacing of the guide thimbles does not permit precision external cutting, either simultaneously or individually.
A variety of devices have been developed over the years for internally severing pipes or the like. Representative of such devices are the ones disclosed and illustrated in U.S. Patents to Baughman (No. 334,928), Mackenzie (No. 672,448), Seymour (No. 837,426) and Buckles (No. 2,130,580). However, such prior art devices have not been suitable nor adapted for use in severing guide thimbles from the top nozzle of an irradiated fuel assembly. Prior art devices fail to address or even recognize the demands encountered while working in the severe and restrictive environment associated with nuclear fuel assemblies: tolerances between interfitting parts must be exceptionally close; introduction of foreign particles into the fuel assembly cannot be tolerated; jamming and binding of parts which slide and rotate relative to one another must be avoided; and all parts must be able to withstand and accommodate distortion due to constant irradiation by radioactive material and corrosive attack by fluids to which they are subjected and in which they are submerged.
Consequently, there is a need for a precision internal tube cutter which alleviates and accommodates the problems associated with nozzle removal in the environment of a nuclear fuel assembly, and thus makes it possible, at less cost and in a rapid, straightforward manner, to reconstitute fuel assemblies which do not incorporate special design features to facilitate such reconstitution.